Ceramics are widely employed in many fields because of excellent electric and magnetic characteristics and biological compatibility as well as mechanical properties such as a heat resistance and an abrasion resistance properties thereof. Among them, a ceramics substrate comprising primarily zirconia has excellent oxygen ion conductivity and heat and corrosion resistance properties and therefore can be effectively applied as a sensor part, an electrolyte film for a fuel cell or a setter for calcination. In order to use ceramics for these applications, they are preferably close ceramics sheets, and therefore fine powders of so-called submicron which are easy to sinter are usually used as a raw material powder. However, the use of fine powders makes it difficult to decompose and remove a binder component and is liable to cause waviness and warp particularly in large and thin sheet moldings because of a large shrinkage generated in sintering process.
A method generally carried out for producing a ceramics sheet is a method in which a slurry comprising a ceramics raw material powder such as alumina, an organic binder and a solvent is molded into a sheet by a doctor blade method, a calendar method or an extrusion method; the resultant sheet is dried to evaporate the solvent and a green sheet is obtained; thereafter the green sheet is arranged to a suitable size by cutting or punching, placed on a setter and calcined to decompose and remove the organic binder; and then, the ceramics powder is sintered.
In general, when a green sheet is treated with heat to prepare a ceramics sheet, it is very difficult to secure an overall and even heat-atmospheric condition (temperature distribution, kinds and concentration of atmospheric gas, flow of atmospheric gas, and the like), and unevenness of the thermal condition is therefore generated at various parts of a sheet, so that warp and waviness are liable to generate. In other words, the presence of a slight difference of degreasing conditions at the respective parts of the sheet prevents the binder from being uniformly removed and generates waviness. A sheet shrinks as it is sintered in calcining, and generating a slight heat atmospheric difference at the respective parts of the sheet results in uneven shrinkage to cause waviness and cracking. In particular, a thin ceramics sheet having thickness of 1 mm or less is light in its own weight, and therefore the sheet itself is easier to be lifted than a conventional thick sheet and more liable to cause waviness. Further, when the respective parts of a sheet moves from the end part to the central part as shrinkage proceeds, the presence of a slight irregularity on a setter or causing abrasion prevents the shrinkage and is easy to cause waviness and cracking.
In the calcination of a sheet having a size of up to a level of 20 square cm (400 cm.sup.2) after sintering, a relatively thin setter with a high density and a high strength can be used. However, in a sheet which is larger than that, a porous thick setter has to be used so that it is not bent even at high temperature, and since the setter becomes insulating and very large in heat capacity, large difference in the temperature is generated between the end and the center of the setter in elevating and lowering the temperature to result in thermal unevenness. Further, when a large sheet is calcined in an electric furnace of a system in which heating is carried out by a heater from the side face and the ceiling or the hearth, the sheet is too large for the furnace, and therefore some part of the sheet is close to the furnace and another part is far there-from, which causes thermal unevenness at the respective parts even in one sheet. Or, a large gas furnace has room in a size of a soaking area in an empty furnace, but the use of a large setter makes it impossible to secure a sufficient path for gas (flame) and is therefore still liable to cause thermal unevenness. These thermal unevenness and prevention of shrinkage are more marked in a sheet having a larger size after sintering and come out notably when the sheet is as large as, for example, exceeding 400 cm.sup.2. In particular, the above descrived tendency becomes marked when the sheet is as large as exceeding 600 cm.sup.2, which causes waviness and warp. Such tendency comes out even to a relatively small-sized sheet having a size of less than 400 cm.sup.2.
Warp and waviness are generated in no small quantities in the thus obtained ceramics sheet even if one green sheet is placed on a setter of one layer and calcined, and larger warp and waviness are generated if, for the purpose of raising the productivity, plural green sheets are superposed one over another on a setter of one layer, and calcined. In particular, such a tendency is marked in the calcination of a green sheet produced using a ceramics powder raw material of submicron. Warp and waviness generated on the ceramics sheet after calcination bring about a locally concentrated stress when a load and a bending force are applied on the said sheet, which causes cracking and breakage. Such a warp and waviness can be reformed by a method in which a sheet is calcined again with a load applied on the sheet. However, cracking and breakage are often generated on the sheet during this reforming process and they can be a large cause of reduction in a yield. And further, it is not preferred from the viewpoint of energy saving that calcination is carried out twice or more.
Accordingly, a method disclosed in, for example, JP-A-6-9268 is proposed as a technique for improving such difficulty. In this method, a ceramics green sheet is calcined with a load applied on the sheet. Employing such a method suppresses warp and waviness at the calcining stage as much as possible and can provide a ceramics sheet having excellent surface flatness. However, the characteristics described above are effectively revealed only when relatively small ceramics sheets are placed on a setter of one layer one by one and calcined. Then, when thin ceramics sheets having a size exceeding 225 cm.sup.2 are calcined for example, plural plates for applying a load are arranged and placed, traces are easy to be marked on the green sheets at the joints of the plates, and therefore it is difficult to suppress waviness and warp sufficiently.
On the other hand, ceramics sheets used for the applications described above have been provided in the form of small sheets having a size falling in a level of 400 cm.sup.2 at the most because of the foregoing reasons, and as the applications thereof are diversified, ceramics sheets which are as thin as 1 mm or less, preferably 0.4 mm or less and as large as 400 cm.sup.2 or more, preferably 600 cm.sup.2 or more are required. However, the existing state is that as described above, it is very difficult to control warp and waviness generated in calcination to lower levels, and the sheets satisfying the requirements of consumers in terms of surface flatness, withstand-load strength and bending characteristic have not yet been obtained.
Also in a sheet having as relatively small size as less than 400 cm.sup.2, slight waviness and warp generated by the causes described above may be a significant cause for deterioration of quality in some cases depending on the applications thereof.